Electrical cooking appliance

ABSTRACT

An electrical cooking appliance has an upper glass-ceramic plate for supporting the cooking vessels to be heated. A heating unit is disposed below the or each heating zone of the plate and comprises a spiral heating element disposed in a supporting bowl. Leak springs outside the bowl act on the heating element by way of cross pieces inside the bowl and press the heating element by a flat surface thereof into good heat-conductive relationship with the underside of the glass-ceramic plate. The heating element is in two parts with a temperature sensing element disposed therebetween, the latter also being pressed against the underside of the plate.

United States Patent [191 Fischer et al.

ELECTRICAL COOKING APPLIANCE Inventors: Karl Fischer; Gerhard Gossler,both of Oberderdingen; Robert Kicherer, Knittlingen, all of Germany12.0.1). Elektr o-Gerate Blane und Fischer, Ober'derdingen, GermanyFiled: Aug. 22, 1972 Appl. No.: 282,701

Assignee:

[30] Foreign Application Priority Data Feb. 3, 1972 Germany 2205132 [56] References Cited UNITED STATES PATENTS 7/1972 l-lurko et a1 219/4621/1972 Dills 219/464 2/1970 Hingorany et al. 219/464 10/1944 Russell219/455 10/1927 Kercher et al.. 219/464 4/1932 Bathrick 219/461 8/1943Tuttle et a1. 219/463 Jan. 29, 1974 2,470,715 5/1949 Olivares 219/464 X3,045,098 7/1962 Norton 219/535 3,348,025 10/1967 Bassett, .11. et al.219/467 2,519,798 7/1970 Walther 219/439 FOREIGN PATENTS OR APPLICATIONS190,179 7/1956 Austria 219/433 277,625 3/1913 Germany 219/438 553,9551/1957 Italy 219/457 Primary Examiner-Volodymyr Y. Mayewsky Attorney,Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond [5 7] ABSTRACT Anelectrical cooking appliance has an upper glassceramic plate forsupporting the cooking vessels to be heated. A heating unit is disposedbelow the or each heating zone of the plate and comprises a spiralheating element disposed in a supporting bowl. Leak springs outside thebowl act on the heating element by way of cross pieces inside the bowland press the heating element by a flat surface thereof into goodheatconductive relationship with the underside of the glass-ceramicplate.

The heating element is in two parts with a temperature sensing elementdisposed therebetween, the latter also being pressed against theunderside of the plate.

25 Claims, 8 Drawing Figures 5 r l7 \h "1 v "Fm f IIIIIIII. r. (7 l K 1447 4 f /7 M 1? PATENTED JAN 2 9 [9?4 SHEET 1 [IF 3 PATENTEB .JAHZS E574SHEET 2 BF 3 PATENTEDJAHZQIW 3,789,189

SHEET 3 BF 3 ELECTRICAL COOKING APPLIANCE The invention relates toanelectrical cooking appliance with an upper plate made from highlyheatresistant glass-like or ceramic material (glass ceramic), on whichcooking vessels or the like are placed, and which can have, ifnecessary, several heated cooking areas, each of which is formed by arespective heating unit provided underneath the plate.

Electrical cooking appliances of this nature are provided to offer thehousewife an even, continuous surface, which is no longer sub-dividedinto individual electrical hotplates, on which cooking vessels can befreely moved about. The glass-ceramic material has, of course, numerousadvantageous properties but it also has a substantial resistance to heattransfer. Certainly with respect to heat radiation in a certainfrequency range it has very favourable transfer values, but it isdifficult to radiate sufficient heat through the plate to enable thecooking area in question to bear comparison with a modern electricalhot-plate. The high heat conductor temperatures required for thismoreover make heavy demands on the heat conductor.

Operation by heat conduction through the glassceramic material hasalready been attempted and suggested, in that heat conductors arepressed directly against the glass-ceramic material. However,difficulties arise here on account of possible fusing of theglassceramic material in the event of localised overheating and onaccount of the glass-ceramic material being undesirably conductive athigher temperatures. Attention must also be paid to the fact thatsaucepans etc. which are moved about will be standing on the plate sothat precautions must be taken to avoid danger if the plate breaks.

A feature of the invention is to provide an electrical cooking applianceof the type mentioned initially which, while being extremely safe touse, enables a high specific heat surface load. at the cooking areas.The heating unit to be provided should be easy to produce and install aswell as sturdy and mechanically sound.

In accordance with the present invention the heating unit comprises asupporting bowl and a heating element disposed in said bowl andresiliently supported thereon for large-area contact with the plate, theheating element comprises a heating wire (tubular heater) which isenclosed by an electrical insulation and a metallic sheath, is flattenedat the side facing the plate and has a small cross section and greatflexibility, and the sheath of the heating element is connectible with aprotective conductor or the like.

A heating element is therefore provided which can be located with alarge area in contact with the glassceramic plate and thus enables heattransmission by heat conduction, which is considerably better than heattransmission by radiation. Nevertheless, the risk of the disadvantageousphenomena associated therewith is substantially avoided. The outersheath of the heating element can be earthed so that safety is assured.

Tubular heaters are of course known. These are generally circular insection and have relatively large section dimensions. They are thereforevery rigid. The tubular heater used in accordance with the invention hasa relatively small cross section and is therefore flexible. When it iswound preferably in a spiral, it should be able to be pressed with aminimum possible force in such a way against the plate that nosubstantial forces are required to obtain substantially full surfacecontact. These slight forces ensure, for example, that when the tubularheater expands during heating it does not cause grating or squeakingnoises at the plate.

The invention is further described, by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a section through a glass-ceramic plate and a heating unit;

FIG. 2 is a top view of the heating unit of FIG. I with theglass-ceramic plate removed;

FIG. 3 is a view of the embodiment of FIG. 1 as seen from below;

FIG. 4 is a detailed section along the line lV-IV of FIG. 1;

FIGS. 5 and 6 are sectional views to a larger scale of details along thelines VV and VIVI of FIG. 4;

FIG. 7 is a diagrammatic illustration of the mounting of the heatingunit on the electric cooking appliance; and

FIG. 8 is a sectional detail of another embodiment of a heating unit.

The heating unit 11 illustrated in FIGS. 1 to 7 is part of an electriccooking appliance whose upper plate 12 is made from highlyheat-resistant, glass-like or ceramic material. This material, which isnormally known as glass-ceramic, is a special, partially ceramicisedglass which is especially noted for its high heat-shock resistance andis already widely used in the making of cooking vessels etc. Theglass-ceramic plate 12 thus forms the actual hot-plate of the electricalcooking appliance, which in other respects may be constructed like aconventional cooker. Several heating units 11 can be disposed under theplate 12.

The heating unit 11 has a supporting bowl 13 which is made from a goodheat-conductive, relatively thick- I walled material, which in theillustrated embodiment is cast aluminum. It has the form of an annularchannel or a semi-torus. A central unheated zone 14 is formed in thecentre of the heating unit. The inner surface 15 of the supporting bowlin. the channel region is made reflective, for example by suitablesurface treatment, while the outer surface 16 is roughened and darklycoloured, for example by colouring or anodizing. What is important,however, is that the outer surface is made as large as possible and hasthe greatest possible radiationabsorption ratio 5 (or a small reflectionratio).

Inner and outer pairs of locating projections 17, 18, which formrespective gaps 19 between one another, protrude from the inner surface15 of the supporting bowl 13. Each gap 19 is somewhat broader than thecorresponding part of a cross piece 20 whose ends are received in thegaps in such a way that it is still able to tilt to a certain extent (inFIG. 4 to the right or left).

As can be seen from FIG. 2, six cross pieces 22 are arranged radially atuniform angular spacings in the embodiment in question. FIG. 4 showsthat each cross piece 20 comprises two adjacent sheet metal parts 21,

particularly in FIGS. 5 and 6, the heating element comprises an innerheating wire 26, which can, for example, be arranged as a spiral,a'conventional electrical insulation 27 in the form of a mineral packingsubstance, in which the heating wire is embedded, and an outer sheath 28made of metal, for example stainless steel. It should be noted that theheating element 23 is basically triangular in cross section, having aflat surface 29 facing the glass-ceramic plate 12 and twodownwardsloping triangle sides 30. Naturally the triangle has veryrounded-off comers. It is preferably so constructed that the flatsurface 29 is substantially longer than the triangle sides 30 so thatthe heating element has the largest possible contact surface against theplate 12.

The heating element is preferably produced as a round tubular heater andgiven the triangular section after production by suitable shaping.Compared with other conventional tubular heaters, the section of theheating element 23 is very small. Preferably its outer dimensionparallel to the plate 12 is less than 5 mm and the height (perpendicularto the plate 12) is less than 4 mm. Since, moreover, the material of thesheath 28 is kept as thin as is admissibly possible, the entire heatingelement is extraordinarily flexible. Flexibility in this instance shouldbe taken to means that the heating element, which is, for example,curved into a spiral, is so inherently movable that its individual coilsare readily displaceable perpendicular to the plane of the spiral. Thisis naturally dependent on the one hand upon the properties and strengthof the materials and upon the section dimensions of the heating elementand the total size of the heating elemen't. The above-mentioneddimensions apply to a heating element of normal size, i.e., a heatingelement which is constructed according to the embodiment and heats acooking area of average size.

As already mentioned, the heating element 23 is divided into twocomponent sections 24, 25, of which the section 24 surrounds theunheated central zone 14 and forms an inner heating ring. The outer end31 of the inner section 24 is bent downwards and, for example,electrically connected to the inner end 32 of the outer section 25. Oneentire turn of the notional spiral defined by the heating element 23 isleft free between the ends 31 and 32, i.e., between the sections 24 and25. A temperature sensor element 33 is located in this free turn of thespiral, the element being likewise in the form of a tube, i.e., theexterior of the sensor element 33 is preferably made of metal andcompletely encloses the sensor disposed inside it. As can be seen fromFIG. 2, this sensor element 33 lies approximately in the centre of thegenerally annular heating surface 34 and is preferably located in thehottest region of this heating surface in order to monitor the peaktemperatures. The sensor element 33 can' operate on various principlesand can be, for example, a resistance wire embedded in an insulatingtube. Preferably, however, a sensor is used of the type described inco-pending US. Pat. application No. 282,700 filed concurrently herewithby the same applicant under the title Temperature sensors. This sensorhas the advantage that the sensor output is not an integral of thetemperature over the entire length of the sensor but localizedoverheating (caused, for example by a cooking vessel placed offcentre)is enough to make the controller connected to the sensor respond. Anintegrating sensor, for example a resistanceor hydraulic sensor, in sucha case of localised overheating would only indicate an average value inits output and would consequently allow localised overheating undercertain circumstances. The advantageous sensor described in theabove-mentioned co-pending application operates with a compactintermediate layer with a temperature-dependent resistance between twoelectrodes.

The sensor element 33 in the form of a single spiral turn is somewhatsmaller in diameter than the heating element thereby forming a somewhatbigger gap between itself and the adjacent turns of the heating element23. In order to obtain the smallest possible heat coupling and one whichremains exactly uniform between the adjacent turns of the heatingelement 23 and the sensor element 33, a spacer plate 35 is providedwhich has a zig-zag or undulating convolution and contains the sensorelement 33 in a central recess, while the outer and inner vertices ofthe spacer plate 35 rest against the sections 24 and 25 of the heatingelement 23.

The sections 24, 25 of the heating element 23 are preferablyelectrically wired directly to one another so that, as can be seen fromFIG, 3, only four terminals lead out from the supporting bowl 13, theseterminals being provided on a connecting piece 36 which is firmlymounted on the supporting bowl. There are two terminals 37 for theheating current and two terminals 38 for the sensor element 33.

The cross pieces 20 which support the heating element 23 and, as can beseen from FIG. 1, the sensor element 33, have already been described.Since the sensor element has a smaller section than the heating element23, it is supported by an upper projection 39' of the sheet metal parts21, 22 of the cross pieces 20. It can also be seen from FIG. 1 that thesensor element preferably has a section giving a greatest possiblecontact surface against the plate 12, this section therefore beingeither triangular, as with the illustrated heating element 23, or in theform of a segment of a circle. The latter form is likewise conceivablefor the heating element 23. I

A spring element 40 in the form of a six-pointed star, as can be seen inFIG. 3, is mounted with a screw 39 inthe central zone 14 of thesupporting bowl, its individual points forming spring arms 41 which, inthe manner of leaf springs, urge the cross pieces 20 upwards towards theplate 12. The free ends 42 of the spring arms 41 project throughapertures 43 in the supporting bowl 13. It is therefore clear that thespring element 40 forms an essentially independent spring suspension foreach individual cross piece 20, that the entire spring element can bedisposed outside the supporting bowl, i.e., in a low-temperature area,and that it can be mounted with a single screw. In order to increase thespring force, the spring arms 41 are stiffened by flanging over theirside edges.

The cross pieces 20 comprise, as already described, two sheet metalparts 21, 22.'On the upper edges of these sheet matal parts, which areconstructed essentially mirror-symmetrically to one another, areprojections 44, which are respectively positioned between two adjacentturns of the heating element 23, and support sections 45 and recessedsections 46 disposed therebetween. The support sections 45 and therecessed sections 46 alternate and are separated by a respectiveprojection 44. It is clear from FIGS. 5 and 6, which show two separatesheet metal parts belonging to one cross piece 20, that where there is asupporting section 45 in a cross piece part 22 supporting and urging oneturn of the heating element 23 towards the plate 12, in the sheet metalpart 21 there is a recessed section which does not rest against thatturn of the heating element 23. In the case of the neighbouring turn itis the exact reverse. It is also clear that the cross piece 20, onaccount of the fact that the gap 19 in the guide projections 17, 18 isbroader than the corresponding thickness of the cross piece, can tilt toa certain extent, so that the cross piece can incline under the pressureexerted on its lower edge by the spring arms 41 in such a way that theheating element 23 is applied in the optimum manner and inaccuracies ofthe cross pieces 20 or the heating elements 23 due to manufacturingtolerances do not have a disturbing effect. This principle can befurther modified and the ideal support would be obtained if eachindividual sheet metal part of a cross piece, i.e., each independentlysprung part rested against the heating element 23 at two points only.

Mention is also made of the fact that the spring force, with which thespring element 40 urges the heating ele ment 23 against the plate 12, isrelatively small. This spring force need only be sufficiently large forit to be capable of distorting the heating element 23 to an extent suchthat the largest possible area is in contact with the plate 12. This isrelatively easy to achieve on account of the great flexibility of theheating element 23. It is also notable that it is advantageous toposition the individual turns of the heating element as close to oneanother as is possible without their touching one another. Preferably,the surface of the heating element 'in contact with the plate 12 shouldrepresent about 80 percent of the total annular heating surface 34.

The outer sheaths 28 of the two parts 24 and of the heating element 23are electrically connected by the sheet metal cross pieces to thesupporting bowl 13 and can be connected to a protective conductor withthe aid of an earth strap 47 connected to the screw 39.

It can be seen from FIG. 3, that two depressions 48 are providedopposite one another in the supporting bowl 13. A projection 49 of afastening yoke 50 (visible in FIG. 7) can engage into each depression48, one end (not shown) of the yoke 50 being pivotably fastened to theframe supporting the plate 12 while the other end 51 is fastened bymeans of a tension spring 52 to a part 53 of this frame.

The tension spring 52 acting by way of the fastening yoke 50 presses theheating unit upwards towards the plate 12 so that the outer edge regionis pressed by way of a resilient intermediate layer 54 against the plate12. Two fastening yokes 50 are provided, each of which engages in arespective one of the depressions 48. The two relevant springs 52 have aspring force which is greater than the force of the spring element 40which urges the cross pieces 20 and therefore the heating element 23 andthe sensor element 33 upwards towards the plate 12.

The embodiment of FIG. 8 mainly corresponds to that of FIGS. 1 to 7. Thesame parts therefore bear the same reference numerals. The heating unit11' has, however, instead of the unitary spring element 40, individualleaf springs 55 which are riveted on the exterior of the supporting bowl13' in the peripheral region and whose free ends 42' project throughapertures 43 into the interior of the supporting bowl 13. The advantageof this embodiment is that the unheated central zone 14' is completelyfree and can be used for other purposes.

It is clear that the springs 52 or another comparable means of pressingthe heating units 11, 11 ensures good contact with the underside of theglass-ceramic plate 12. The spring pressure in conjunction with theresilient intermediate layer 54 ensures that no disturbing squeakingnoises occur during heat-conditioned movements. The forces of thesprings 52 overcome the force of the spring part 40 or the sum of theforces of the leaf springs 55. The heating element 23 and the sensorelement 33 are pressed with uniform force, the spring force of thespring part 40 or the leaf springs 55 being sufficiently great toguarantee a large-area contact of the flat surfaces 29 against the plate12. In operation, i.e., when current is passed through the heating wire26, a relatively direct heat bridge is provided by way of the insulation27, the sheath 28 and the plate 12 to the cooking vessel standing on theplate. The percentage of the total heat which flows off by way of therelatively large flat surface 29 and the glassceramic plate 12 as aresult of heat conduction is very high. The sheath 28 of the heatingelement 23 also contributes in that it conducts the heat flowing fromthe heating wire 26 to the underside upwards, i.e., towards the regionof the flat surface 29. The heat which nevertheless radiates from thetriangle sides 30 at the underside of the heating element is reflectedby the reflective inner surface 15 of the supporting bowl 13. As alreadydescribed, the supporting bowl 13 is made from relatively strong-walled,good heat-conductive material. Thus, localised overheating does notoccur but the radiant energy which is not reflected can be welldistributed in the supporting bowl. Overheating is also pre vented bytreating the outer surface 16 of the supporting bowl to promoteradiation. It should be noted here that the losses incurred by radiationor convection at the outer surface 16 of the supporting bowl 13 areextra-ordinarily low. They amount to approximately one or less percentof the total output. The difficulties with the reflector, which arosepreviously, lay only in the fact that the reflector could not dissipateeven the relatively small amounts of heat absorbed by it and thus tendedto overheat.

The arrangement of the sensor in the described manner is particularlyadvantageous. It can be used in conjunction with known regulators orprotective switches for temperature control or overheating protection.As an overheating protection this arrangement is especially advantageoussince the sensor element 33 is disposed in the hottest region and readsdirectly the critical temperature, namely the temperature at theunderside of the glass-ceramic plate, whose sustained temperaturestability lies between 600 and 700C. The two sections 24, 25 can beconnected in parallel or in series. However, they are preferablyconnected in parallel in order to be able to use a heating wire adaptedto the sheath diameter. One of the greatest advantages of the inventionis that an easily manipulated heating unit is provided which can beattached by means of the fastening yokes 50 and few manipulations on thesupporting frame of the glass-ceramic plate 12, on which a connectingpiece with the terminals is firmly mounted and which is also easilyreplaceable when in need of repair. The flexible tubular heaters with asmall cross section, which are used in the invention, have the advantageof a very low thermal capacity so that the electrical cooking appliancereacts extremely quickly to regulation or control. It is alsoadvantageous that the sensor and its terminals are directly included inthe heating unit and, apart from establishing electrical connections, nofurther connections are necessary. The sensor does not determine thestate to be measured at a certain point but over an area in an entireregion which represents a genuine representative cross section.

We claim:

1. In an electrical cooking appliance, the combination comprising:

a highly heat-shock resistant, glass-ceramic upper plate having at leastone cooking zone for receiving and supporting a cooking vessel to beheated;

a separate heating unit mounted beneath said plate at each cooking zone,each unit including (1) a supporting bowl, (2) a heating elementdisposed in said bowl adjacent said plate and having a metallic sheath,a heating wire enclosed in said sheath and an electrical insulationmaterial between said heating wire and said sheath, said sheath beingflattened at the side adjacent said plate to provide a direct areacontact between said heating element and said plate, said heatingelement having a small cross-section and being flexible for heatconductive contact with said plate, and (3) means for supporting saidheating element within said supporting bowl for direct contact of saidheating element with said plate; and

means for detachably mounting said heating unit below said plateincluding spring means for urging said heating unit support bowl againstsaid plate and resilient means operative between said bowl and saidplate, said heating element being resiliently pressed into heatconductive contact with said plate whereby each separate heating unitmay be independently installed or removed from beneath said upper plate.

2. An electrical cooking appliance according to claim 1, which furthercomprises protective conductor means connected to said sheath.

3. An electrical cooking appliance according to claim 1, in which saidsupporting bowl has an interior surface formed as a reflector.

4. An electrical cooking appliance according to claim 1, in which saidheating element direct area contact occupies approximately 80 percent ofsaid cooking zone.

5. An electrical cooking appliance according to claim 1, in which saidheating element is wound in the form of a spiral.

6. An electrical cooking appliance according to claim 1, in which saidheating element has a generally triangular cross section.

7. An electrical cooking appliance according to claim 1, in which saidheating element has in cross section outer dimensions of less than 5 mmparallel to said plate and of less than 4 mm perpendicular to saidplate.

8. An electrical cooking appliance according to claim 1, in which saidsupporting bowl is circular and defines an annular channel encircling anunheated central zone of the heating unit.

9. An electrical cooking appliance according to claim 3, in which saidsupporting bowl comprises good heatconductive material and its wallthickness is high in order that its temperature shall be substantiallyuniform.

10. An electrical cooking appliance according to claim 1, in which saidsupporting bowl has an outer surface formed as a non-reflector so thatit shall have the greatest possible radiation-absorption ratio 6.

11. An electrical cooking appliance according to claim 10, in which saidouter surface of said supporting bowl is roughened.

12. An electrical cooking appliance according to claim 11}, in whichsaid outer surface of said supporting bowl is darkly coloured.

13. An electrical cooking appliance according to claim 10, in which saidouter surface of said supporting bowl is oxidized.

14. An electrical cooking appliance according to claim 1, in which saidheating element support means includes a plurality of cross piecesdisposed in said bowl and supporting said heating element, and resilientspring means acting individually against said cross pieces to urge saidheating element against said plate.

15. An electrical cooking appliance according to claim 14, in which saidheating element is wound in the form of a spiral and said cross piecesextend generally radially with respect to said spiral and in which eachof said cross pieces comprises two adjacent generally vertical sheetmetal parts having edges facing the heating element so formed that eachsheet metal part supports every other turn of the heating element andevery turn of the heating element is supported by a respective one onlyof the two sheet metal parts.

16. An electrical cooking appliance according to claim 14, in whichguide means are provided in said supporting bowl and loosely locate saidcross pieces so that the latter can tilt to a limited extent.

17. An electrical cooking appliance according to claim 14, in which saidresilient spring means is adapted to exert a spring force urging saidheating element against said plate with a pressure not substantially inexcess of that necessary for large-area contact of said heating elementwith said plate.

18. An electrical cooking appliance according to claim 14, in which saidresilient spring means comprise a plurality of leaf springs fastened tosaid supporting bowl and acting individually on said cross pieces.

19. An electrical cooking appliance according to claim 18, in which saidresilient spring means comprises a coherent, star-shaped spring partspring material, which is attached centrally to said supporting bowl atthe outside thereof and has arms which define said leaf springs, saidbowl having apertures and said arms having free ends which extendthrough said apertures and individually engage said cross pieces.

20. An electrical cooking appliance according to claim 18, in which saidleaf springs are mounted individually on said supporting bowl in theperipheral region of the outer surface of the latter and in which saidsupporting bowl has apertures, said leaf springs having free ends whichextend through said apertures and individually engage said cross pieces.

21. An electrical cooking appliance according to claim 26, in which saidresilient means comprises a resilient intermediate layer disposedbetween the outer edge region of said supporting bowl and the undersideof said plate. i

22. An electrical cooking appliance according to claim 5, in which saidspiral wound heating element is in two parts leaving free a spacebetween said heating element parts and in which a temperature sensorelement is disposed in said free space.

23. An electrical cooking appliance according to claim 22, in which saidfree space is formed by a turn of the notional spiral defined by saidheating element and in which said temperature sensor element is curvedinto the shape of a spiral turn and pressed resiliently against theunderside of said plate.

24. An electrical cooking appliance according to claim 23, in which anundulating spacer plate is disposed in said free space to maintain saidsensor element at a uniform distance from the adjacent turns of saidheating element.

25. In an electrical cooking appliance, the combination comprising:

a highly heat-shock resistant, glass-ceramic upper plate having at leastone cooking zone for receiving and supporting a cooking vessel to beheated;

a separate heating unit mounted beneath said plate at each cooking zone,each unit including (1) a supporting bowl, (2) a heating elementdisposed in means for detachably mounting said heating unit below saidplate including spring means for urging said heating unit support bowlagainst said plate, said heating element being resiliently pressed intoheat conductive contact with said plate whereby each separate heatingunit may be independently installed or removed from beneath said upperplate.

' Po-ww UNITED STATES PATENT OFFICE (s/sq) CERTIFICATE OF CORRECTIONPatent No. 317391189 Dated January 2 1974 Invgntor( Karl Fischer et al.I

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Assignee change "E.G.D." t0 --E.G.O.--;

PrioritzData change "22651329 ro -P2 205l32.7-;

Column 3, line 25 change. "means" to "mean- Column 8, line 61 change"claim 26" to --claim l--.

Signed and sealed this 18th day of June 19114.;

(SEAL) Attest:

EDWARD H.F'LETOHER,JR. c. MARSHALL DANN Attesting Officer IOommiasionerpf Patents *zgxgg UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. ,1 9 Dated January 29, 1974 Inventor-(Karl-Fischer et a1.

Tt is certified that error appears in the above-identified patent vandthat said Letters Patent are hereby corrected as shown below:

Assi riee chan e "E.G.D." to E.G.O.--;

Prior itfData change "zzdsls z' tQ -P2 205l32.7--

Column; 3, line 25 change. Fmeans" v to mean--;

Co lfim n" 8} ,line 61 change ,claim 26" to -claim 1--.

Signed and sealed this 18th day of June' 197a."

(SEAIJ Attes't:

C MARSHALL DANN EDWARD umnmclmmml Attesting Officer, v e Cgmmi ssionerpfPatents

1. In an electrical cooking appliance, the combination comprising: ahighly heat-shock resistant, glass-ceramic upper plate having at leastone cooking zone for receiving and supporting a cooking vessel to beheated; a separate heating unit mounted beneath said plate at eachcooking zone, each unit including (1) a supporting bowl, (2) a heatingelement disposed in said bowl adjacent said plate and having a metallicsheath, a heating wire enclosed in said sheath and an electricalinsulation material between said heating wire and said sheath, saidsheath being flattened at the side adjacent said plate to provide adirect area contact between said heating element and said pLate, saidheating element having a small cross-section and being flexible for heatconductive contact with said plate, and (3) means for supporting saidheating element within said supporting bowl for direct contact of saidheating element with said plate; and means for detachably mounting saidheating unit below said plate including spring means for urging saidheating unit support bowl against said plate and resilient meansoperative between said bowl and said plate, said heating element beingresiliently pressed into heat conductive contact with said plate wherebyeach separate heating unit may be independently installed or removedfrom beneath said upper plate.
 2. An electrical cooking applianceaccording to claim 1, which further comprises protective conductor meansconnected to said sheath.
 3. An electrical cooking appliance accordingto claim 1, in which said supporting bowl has an interior surface formedas a reflector.
 4. An electrical cooking appliance according to claim 1,in which said heating element direct area contact occupies approximately80 percent of said cooking zone.
 5. An electrical cooking applianceaccording to claim 1, in which said heating element is wound in the formof a spiral.
 6. An electrical cooking appliance according to claim 1, inwhich said heating element has a generally triangular cross section. 7.An electrical cooking appliance according to claim 1, in which saidheating element has in cross section outer dimensions of less than 5 mmparallel to said plate and of less than 4 mm perpendicular to saidplate.
 8. An electrical cooking appliance according to claim 1, in whichsaid supporting bowl is circular and defines an annular channelencircling an unheated central zone of the heating unit.
 9. Anelectrical cooking appliance according to claim 3, in which saidsupporting bowl comprises good heat-conductive material and its wallthickness is high in order that its temperature shall be substantiallyuniform.
 10. An electrical cooking appliance according to claim 1, inwhich said supporting bowl has an outer surface formed as anon-reflector so that it shall have the greatest possibleradiation-absorption ratio epsilon .
 11. An electrical cooking applianceaccording to claim 10, in which said outer surface of said supportingbowl is roughened.
 12. An electrical cooking appliance according toclaim 10, in which said outer surface of said supporting bowl is darklycoloured.
 13. An electrical cooking appliance according to claim 10, inwhich said outer surface of said supporting bowl is oxidized.
 14. Anelectrical cooking appliance according to claim 1, in which said heatingelement support means includes a plurality of cross pieces disposed insaid bowl and supporting said heating element, and resilient springmeans acting individually against said cross pieces to urge said heatingelement against said plate.
 15. An electrical cooking applianceaccording to claim 14, in which said heating element is wound in theform of a spiral and said cross pieces extend generally radially withrespect to said spiral and in which each of said cross pieces comprisestwo adjacent generally vertical sheet metal parts having edges facingthe heating element so formed that each sheet metal part supports everyother turn of the heating element and every turn of the heating elementis supported by a respective one only of the two sheet metal parts. 16.An electrical cooking appliance according to claim 14, in which guidemeans are provided in said supporting bowl and loosely locate said crosspieces so that the latter can tilt to a limited extent.
 17. Anelectrical cooking appliance according to claim 14, in which saidresilient spring means is adapted to exert a spring force urging saidheating element against said plate with a pressure not substantially inexcess of that necessary for large-area contact of said heating elementwith said plate.
 18. An electrical cooking appliance according to claim14, in which said resilient spring means comprise a plurality of leafsprings fastened to said supporting bowl and acting individually on saidcross pieces.
 19. An electrical cooking appliance according to claim 18,in which said resilient spring means comprises a coherent, star-shapedspring part spring material, which is attached centrally to saidsupporting bowl at the outside thereof and has arms which define saidleaf springs, said bowl having apertures and said arms having free endswhich extend through said apertures and individually engage said crosspieces.
 20. An electrical cooking appliance according to claim 18, inwhich said leaf springs are mounted individually on said supporting bowlin the peripheral region of the outer surface of the latter and in whichsaid supporting bowl has apertures, said leaf springs having free endswhich extend through said apertures and individually engage said crosspieces.
 21. An electrical cooking appliance according to claim 26, inwhich said resilient means comprises a resilient intermediate layerdisposed between the outer edge region of said supporting bowl and theunderside of said plate.
 22. An electrical cooking appliance accordingto claim 5, in which said spiral wound heating element is in two partsleaving free a space between said heating element parts and in which atemperature sensor element is disposed in said free space.
 23. Anelectrical cooking appliance according to claim 22, in which said freespace is formed by a turn of the notional spiral defined by said heatingelement and in which said temperature sensor element is curved into theshape of a spiral turn and pressed resiliently against the underside ofsaid plate.
 24. An electrical cooking appliance according to claim 23,in which an undulating spacer plate is disposed in said free space tomaintain said sensor element at a uniform distance from the adjacentturns of said heating element.
 25. In an electrical cooking appliance,the combination comprising: a highly heat-shock resistant, glass-ceramicupper plate having at least one cooking zone for receiving andsupporting a cooking vessel to be heated; a separate heating unitmounted beneath said plate at each cooking zone, each unit including (1)a supporting bowl, (2) a heating element disposed in said bowl adjacentsaid plate and having a metallic sheath, a heating wire enclosed in saidsheath and an electrical insulation material between said heating wireand said sheath, said sheath being flattened at the side adjacent saidplate to provide a direct area contact between said heating element andsaid plate, said heating element being flexible and having a smallcross-section, and (3) means for resiliently supporting said heatingelement within said supporting bowl for resiliently biased directcontact of said heating element with said plate; and means fordetachably mounting said heating unit below said plate including springmeans for urging said heating unit support bowl against said plate, saidheating element being resiliently pressed into heat conductive contactwith said plate whereby each separate heating unit may be independentlyinstalled or removed from beneath said upper plate.